Electrophotosensitive copiers include a photoconductor with a photoconductive layer with a conductive backing. The photoconductor is transported along an endless path relative to a plurality of work stations, each of which is operative when actuated to perform a work operation on the electrophotosensitive medium. Such stations include a charging station at which a uniform charge is placed on the photoconductive layer, an exposure station at which the charged photoconductive layer is image-wise exposed to actinic radiation from the medium to create an electrostatic image of the medium in the photoconductive layer, a developing station at which the electrostatic image is contacted with finely divided charged toner particles for adhering to the photoconductive layer in a configuration defined by the electrostatic image, a transfer station at which such toner particles are transferred in the image configuration to a receiving surface, and a cleaning station at which residual toner is removed from the photoconductive layer so that it can be reused.
One of the most effective forms for transfer station has been fusing rollers. Frequently, at least one of the rollers is heated so that both heat and pressure are applied to toner particles at the nip between the rollers to fix the toner to a receiver sheet in an image-wise configuration.
Fuser rollers performance reliability is one of the most important factors which influences the electrophotographic copier life cycle cost and customer satisfaction. Fuser rollers, when heated externally via a heater roller, are affected by the ability of the heater roller to remain free of contamination.
Known to the electrostatographic fixing art are various fuser members adapted to apply heat and pressure to a heat-softenable electrostatographic toner on a receiver, such as paper, to permanently fuse the toner to the receiver rollers, fuser plates and fuser belts for use in fuser systems such as fuser roller systems, fuser plate systems and fuser belt systems. The term "fuser roller" is used herein to identify one of the elements of a fusing system. Commonly, the fuser roller is a fuser roller or pressure roller and the discussion herein may refer to a fuser roller or pressure roller. However, often the fuser roller is heated so that pressure and heat is applied to fuse a toner image.
One of the long-standing problems with electrostatographic toner fusing mechanism is the adhesion of the heat-softened toner particles to the surface of a fuser member and not to the receiver, known as "offset" which occurs when the toner-bearing receiver is passed through a fuser system. There have been several approaches to decrease the amount of toner offset onto fuser members. One approach has been to make the toner-contacting surface of a fuser member, for example, a fuser roller and/or pressure roller of a non-adhesive (non-stick) material.
One known non-adhesive coating for fuser members comprises fluoropolymer resins, but fluoropolymer resins are non-compliant. It is desirable to have compliant fuser members to increase the contact area between a fuser member and the toner-bearing receiver. However, fuser members with a single compliant rubber layer absorb release oils and degrade in a short time leading to wrinkling artifacts, non-uniform nip width and toner offset. To make fluoropolymer resin coated fuser members with a compliant layer, U.S. Pat. Nos. 3,435,500 and 4,789,565 disclose a fluoropolymer resin layer sintered to a silicone rubber layer which is adhered to a metal core. In U.S. Pat. No. 4,789,565, an aqueous solution of fluoropolymer resin powder is sintered to the silicone rubber layer. In U.S. Pat. No. 3,435,500, a fluoropolymer resin sleeve is sintered to the silicone rubber layer. Sintering of the fluoropolymer resin layer is usually accomplished by heating the coated fuser members to temperatures of approximately 500.degree. C. Such high temperatures can have a detrimental effect on the silicone rubber layer causing the silicone rubber to smoke or depolymerize, which decreases the durability of the silicone rubbers and the adhesion strength between the silicone rubber layer and the fluoropolymer resin layer. Attempts to avoid the detrimental effect of the high sintering temperatures that have on the silicone rubber layer have been made by using dielectric heating of the fluoropolymer resin layer, for example, see U.S. Pat. Nos. 5,011,401 and 5,153,660. Dielectric heating is, however, complicated and expensive and the fluoropolymer resin layer may still delaminate from the silicone rubber layer when the fuser members are used in high pressure fuser systems. In addition, a fuser member made with a fluoropolymer resin sleeve layer possess poor abrasion resistance and poor heat resistance.
When a fuser roller becomes contaminated with toner, the contamination can be transferred to the heater roller, thus contributing to the failure. The contamination of the fuser roller can also contribute to the paper jam failure in an electrophotographic engine and it is primarily due largely to the inability of the paper itself to release from the fusing roller. The toner release from the rollers in the electrophotographic engine, which commonly termed as "offset", is sometimes aided by applying a suitable oil on the roller surface. A probable mechanism for the reduction in offset with oil is that oil flowing into the pores on the surface of the roller material forms a barrier. This barrier is an aid in retarding the offset of contamination. The viscosity of various oils and their molecular structure determine whether the oils will be adsorbed to the roller surfaces. The increase in viscosity makes the displacement of oil from the roller surface more difficult. If a high molecular weight oil is adsorbed on the roller surface, entropy considerations suggest that such oil molecules will have to detach from several sites simultaneously, to regain mobility, making detachment less probable.
The surface energy of the roller material, and its surface morphology can also influence the degree of toner release (offset). Other important material properties such as, wear and abrasion characteristics, thermal conductivity and reactivity or bonding with various functional oils, which are normally used to reduce the offset characteristics can contribute to the choice for suitable heater and pressure roller materials. The commonly used roller material such as elastomers and other experimental roller such as hardcoat anodized rollers, teflon, electroless nickel and electroless nickel impregnated with teflon lack one or more shortcomings in material characteristics described above, and contribute to varying degrees of toner offset.